The present invention relates to control of the electrical energy delivered by a source to a load and, more particularly, to novel multiplexing apparatus for reducing the number of parts and interconnections required between at least one power-controlling switching means, in series with the load and the source, and control circuitry for properly actuating the at least one switching means.
It is known to use phase-control circuitry to control the power consumed, from an AC source, by a load. This form of power-control circuit connects the load across the source for only a portion of each source waveform half-cycle. It is known to control the duration of the load connection time interval responsive to either an external parameter, such as a variable resistance in a lamp dimmer circuit, which external parameter is not influenced by any physical characteristic of the load (an open-loop control circuit), or to feed back information from the load to obtain a self-correcting response (a closed-loop control circuit). It is also known to configure either an open-loop or closed-loop load control circuit with at least one power-switching solid-state device in series with the load, across the source, and to operate the at least one power-switching device in either a forward phase-control condition or a reverse phase-control condition. In the forward-phase-control condition, the power-switching device: is in the non-conductive condition for some time after each line voltage zero crossing at which a particular line voltage waveform half-cycle commences; is controlled to the conducting condition ("turned-on") at a time during that line voltage waveform half-cycle when a non-zero line-line voltage is present; and is removed from conduction ("turnedoff") at the next line-line voltage zero crossing. The termination of conduction can be a self-commutation action (if the power-switching device(s) is an SCR or the like device) or can be a driven turn-off action responsive to a turn-off signal provided by a control circuit external to the device(s) (if each of the at least one power-switching device is a controllable-turn-off device, such as an insulated gate transistor (IGT), a bipolar junction transistor, a power MOSFET and the like). In the reverse-phase-control condition, the at least one power-switching device (as more fully described and claimed in my co-pending U.S patent application Ser. No. 529,296, filed Sept. 6, 1983, now U.S. Pat. No. 4,528,494, issued July 9, 1985, assigned to the assignee of the present application and incorporated herein in its entirety by reference): is controlled to the conducting condition at each line voltage zero crossing; remains conductive for a time interval thereafter (responsive to an open-loop or closed-loop input); is removed from conduction thereafter while a non-zero voltage is present across the load; and remains non-conductive until the next line voltage zero crossing occurs. It will be understood that only power-switching devices capable of being driven to the turned-off condition can be utilized for in reverse-phase-control circuits.
Either form of load control circuit at least requires: (a) means for determining the occurrence of line voltage zero crossings, which zero-crossing means requires a pair of input signals to determine the positive-going zero crossing and the negative-going zero crossing of the line voltage waveform; (b) means for connecting to at least one of the switching device(s) the turn-on and/or turn-off signal(s); and (c) a circuit common connection. In order to provide greatest reliability and lowest cost to the user, it is desirable to reduce to a minimum both the number of components and the number of interconnections, between the control circuitry and the load/switching device(s) combination. For similar reasons, it is also desirable to provide the control circuitry in an integrated circuit form. However, it will be understood that providing closed-loop control of the load is also desirable, particularly where the load may be prone to accelerated failure if a particular load parameter magnitude is exceeded; feedback control of the load parameter typically requires at least one additional interconnection between the load and/or switching device and the switching device control circuit itself. It is also highly desirable to provide operating power to the control circuit from the line itself, necessitating at least one (and typically a plurality of) additional interconnections. Other required and/or desired features may further increase the number of interconnections between the load/switching device(s) and the switching control circuit: a reduced switching voltage rate-of-change (dV/dt) may be required, e.g. to reduce electromagnetic interference caused by the switching process, to prevent exceeding a switching device limit (responsible for accelerated failure of the switching device) and the like; monitoring of the switching device controlled-conduction-circuit voltage and/or current may be required to assure that the turned-on switching device is in the saturated condition, and thus not dissipating excessive power which may cause an accelerated failure rate; and the like. For example, the controlled switching of non-regenerative power semiconductors may be provided as described and claimed in my co-pending U.S. patent application Ser. No. 499,579, now U.S. Pat. No. 4,540,893, issued Sept. 10, 1985, and the prevention of excess power dissipation in power switching semiconductors may be provided as described and claimed in my co-pending U.S patent application Ser. No. 499,590, now U.S. Pat. No. 4,547,828, issued Oct. 15, 1985, assigned to the assignee of the present application, and incorporated herein in their entireties by reference. When the power-switching semiconductors(s) control circuitry includes any such additional features, the number of interconnections therebetween, and the number of discrete components (resistors, capacitors and the like) required, can be significant. The additional cost, and especially the additional failure rates thereof, may predominately determine the cost and/or reliability of the entire load control apparatus. For example, a closed loop lamp control circuit, having an integrated circuit drive means for a pair of power-switching semiconductors, can require as many as 10 external resistors and 12 interconnections for completing the circuit. It is highly desirable to provide some means for reducing the number of components external to the switching device control (integrated) circuit, and for reducing the number of interconnections between that control (integrated) circuit and the power-switching device(s) and load.